In most automobiles of recent vintage the passenger compartment is supplied with air by a blower having an intake or grille which opens upwardly, in front of the windshield. This intake, which is sometimes under the hood itself, is separated from the engine compartment by a so-called hood gasket, which usually engages the underside or edge of the hood between the inlet and the engine. The purpose of the hood gasket is to reduce the intake by the blower of heat, humidity, and potentially harmful fumes (including particulates) from the engine compartment. (As used herein the term "engine compartment fumes" is meant to include all gases coming from or through the engine compartment, radiator cooling air, entrained dust, particles of brake composition from the wheel wells, exhaust from preceding vehicles, and the like.)
If the blower intake is near or on the centerline of the vehicle, the intake is usually sufficiently well isolated from the engine compartment by the hood gasket that the blower receives air from the relatively "clean" space above the hood or cowl and in front of the windshield, and which is not contaminated with engine compartment fumes.
However, I have observed that where the blower intake is located near the curb- or "off-side" of the vehicle (as it is for example in downsized General Motors rear wheel drive cars made in 1977-1985 and having B, C, D and AG type bodies), the blower intakes engine compartment fumes more frequently. Such fumes escape from the engine compartment around the end of the hood gasket, and in part are swept into the blower intake, from which they are circulated into the passenger compartment. Moreover, under certain wind conditions, or where there are obstructions adjacent the vehicle or the tailpipe, the exhaust gas "plume" from a curbside tailpipe will travel forwardly along the body and will contaminate air entering the intake. (In cold weather this plume can be traced as "white smoke.")
I have further observed that when such a vehicle is stopping or has just stopped, dust particles in its wake can sometimes continue to move forwardly along the side of the car, across the window post, and can be pulled into the blower intake from the curb side. This flow path can be seen when a car traveling on a dusty road stops: the dust wake travels forward along the sides of the car and curves across the corners of the hood, into the blower intake and the radiator. (As used herein, the term "exhaust fumes" is meant to include dust wake as well as tailpipe gas.)
Clearly, the direct circulation by the blower into the passenger compartment of engine compartment or exhaust fumes, indeed of any air except clean outside air, is uncomfortable at the very least, may contribute to unintentional carbon monoxide intake, and may be potentially harmful.